Naturalist John Muir called them Lunar Spraybows. Some people call them lunar rainbows, but the most common name is "moonbow." No matter what name you use, it's an odd-sounding phenomenon with a simple enough explanation.
We've all seen daytime rainbows created when sunlight hits a mist of water in the air. In fact, it's not uncommon to see that kind of rainbow at Yosemite Falls in the spray at its base in spring and early summer. In fact, we have a picture of that, too just a few pages further along in this Yosemite Falls picture gallery.
A few times each year, moonlight and mist conspire to form a Yosemite Falls rainbow at night, too. It takes the right conditions: enough mist and the correct placement of viewer and the moon, along with clear, dark skies and bright moonlight. Yosemite moonbows appear in 3- to 4-day stretches up to four times a year, usually in late spring and early summer. They're brightest when the moon is 100% full, which is also the day it rises shortly after sunset.
Yosemite Falls Moonbow
The most common places to see a Yosemite moonbow are near the base of Yosemite Falls (from the viewing terrace at the west end of the bridge) and from Cook's Meadow near Sentinel Bridge, but it can also show up from other spots in the Yosemite Valley where you can see the falls.
Don't expect to see the beautiful colors that show up in this photo, though. When it gets dark, human eyes lose the ability to see the colors that a camera's sensor (or film) records. What you'll see instead (at best) is a silvery-white glow.
This occurrence is extremely popular with photographers and on the night we took this image on an April Thursday evening, we estimated at least 150 of them had tripods and cameras set up in the area.
If you want to see or photograph a Yosemite moonbow, check the moonbow date predictions.
The rest of this gallery shows Yosemite Falls in a variety of conditions, photographed over a period of years, from bone dry to gushing with water and even frozen solid.
Yosemite Falls in Spring
Water flow varies each year, depending on how much snow is melting in the high mountains. 2010 was a spectacular year, with more water than had been seen in quite a few years. The roar of Yosemite Falls sounded like a locomotive passing through the valley and it was easy to get quite wet even if you were standing a distance away.
If you want to see more of Yosemite in its most beautiful season, check the gallery of Yosemite in Spring Photos.
Dry Yosemite Falls
Peak runoff typically occurs in May or June and by early autumn, the water flow often slows to just a trickle. Yosemite Creek, which creates the falls is ephemeral like many other High Sierra streams, only existing for a short period of time following precipitation or snow melt.
From the valley floor, Yosemite Falls looks like two separate falls, but from this viewpoint, it's easy to see that it's really just one waterfall that takes a detour on the way down. The total drop is 2,425 feet, making it one of the world's tallest waterfalls.
Yosemite Falls Rainbow
The year this image was taken, the mist at the base of the upper fall conspired with the sunshine to create a rainbow. In a particularly wet year, Yosemite Creek may keep flowing all year, but this much water was more likely the result of an early rainfall.
From a scientist's point of view, the same natural laws created this rainbow and the moonbow on the first page of this gallery, but the effect is quite different.
Frozen Yosemite Falls
On a cold, winter morning, the spray from Yosemite Falls is sometimes frozen, until the sun touches it. As it melts, you can hear cracking noises and see parts of the ice breaking off. Those who know the place well can often guess how the previous night's low temperature by looking at the amount of ice alongside the Upper Fall in the morning: the more ice, the colder the night.
You might see a scene list this if you follow a winter storm into Yosemite Valley. You can see more photos in the Yosemite in Winter Gallery.
Yosemite Falls may be the most famous waterfall at Yosemite, but it's not the only one. In fact, some of the others are world famous, too. You can find more about them in the Yosemite Waterfall Guide.